1. Field of the Invention
The present invention relates to a developing apparatus, a developing method, an image forming apparatus, and an image forming method using electrophotography.
2. Description of the Related Art
Higher quality of electrophotographic images has been required, and suppression of generation of “fogging” caused by toners adhering to non-image portions of electrophotographic images has been required.
A reduction in size of image forming apparatuses using electrophotography has been required by users in the market.
One of methods for achieving the reduction in size is systems (hereinafter also referred to as a “cleaner-less system”) without a cleaner for removing toners remaining on the surfaces of electrophotographic photosensitive members, such as cleaning blades, (hereinafter also simply referred to as a “cleaner”). Unfortunately, in the cleaner-less system, the role of the cleaner needs to be complemented by another member.
Namely, the image forming apparatuses employing the cleaner-less system have no cleaner for the electrophotographic photosensitive member, and the charging member may be contaminated by a toner adhering to the charging member if the toner adheres to non-developed portions in the electrophotographic photosensitive member, such as portions in which no electrostatic latent image is formed, and are not transferred onto paper. Accordingly, to suppress contamination of the charging member, the developing apparatus should more significantly suppress adhesion of the toner to non-developed portions in the electrophotographic photosensitive member (hereinafter also referred to as “fogging”). The adhesion of the toner to non-image portions is caused by a developer carried on a developing carrying member if the developer contains part of the toner charged at a polarity opposite to a target polarity (hereinafter also referred to as “reverse toner”) and part of the toner having an absolutely insufficient charge amount although charged at a target polarity.
Japanese Patent Application Laid-Open No. 2004-333682 suggests a developing carrying member including a positive-charging resin layer to enhance a negative charging ability to a toner.
The present inventors have examined and found that the developing carrying member described in Japanese Patent Application Laid-Open No. 2004-333682 can sufficiently negatively charge the toners even under a high temperature and high humidity environment in particular in which the charge amount of the toner readily become insufficient.
Unfortunately, such a developing carrying member will provide an excessive charge amount of the toner (charge up) under a low temperature and low humidity environment. In this case, the charged up toner adheres to the surface of the developing carrying member, preventing a toner newly fed to the developing carrying member from being sufficiently charged. Such a toner having an insufficient charge amount may cause fogging.
Moreover, a toner and paper powder adhering onto the electrophotographic photosensitive member may cause insufficient Vback to generate defects in electrophotographic images. If the Vback is increased to prevent this phenomenon, the reverse toner more readily adheres to non-developed portions to cause more fogging.